MANAMA, Bahrain â€” The king of Bahrain declared a three-month state of emergency on Tuesday as more than 10,000 protesters marched on the Saudi Arabian embassy here to denounce a military intervention by Persian Gulf countries the day before.

The entrance of foreign forces, including Saudi troops and those from other Persian Gulf countries, threatened to escalate a local political conflict into a regional showdown; on Tuesday, Tehran , which has long claimed that Bahrain is historically a part of Iran, branded the move â€œunacceptable.â€

The foreign troops did not appear to take up positions in the capital, Manama, early on Tuesday, heading instead for the palace neighborhood of Riffa.

Nevertheless, the city took on the feel of a ghost town: businesses were shuttered, malls closed and the streets largely deserted as residents watched apprehensively to see what role the foreign military would play to quell widespread demonstrations.

Since a series of violent clashes on Sunday, the authorities have appeared to hand over whole sections of Manama and surrounding villages to the protesters, who have blocked roads with rubble and dumpsters.

Outside the activist stronghold of Sitra on Tuesday, protesters set up checkpoints and clashed with security forces, who fired tear gas but did not enter the village.

By the early afternoon in Manama, a large group of protesters had gathered in the central Pearl Square â€” the emblem of the Bahrain protests, much as Cairoâ€™s Tahrir Square assumed symbolic significance in the Egyptian uprising â€” and began marching to the Saudi embassy.

Bahrainâ€™s Ministry of Information, which has recently used its Twitter account to announce road closures, posted two messages in close succession claiming that protesters had attacked police and security forces in villages around the capital. One message suggested that a security officer had died: â€œA member of security force passed away in Maameer this evening when deliberately run over by one of the rioters.â€

Another said: â€œPolice patrols exposed to shooting by automatic weapons by a group of vandals in Buri. No injuries.â€

While there were no reports of fighting between protesters and foreign troops so far, The Associated Press, citing an unnamed Saudi security official, said that one soldier had been shot and killed by a protester on Tuesday. No details were given and the report could not be immediately confirmed.

The decision by Saudi Arabia to send in troops on Monday threatened to further inflame the conflict and transform this teardrop of a nation in the Persian Gulf into the Middle Eastâ€™s a proxy battlefield between regional and global powers. On Monday, Iranian state-run media went so far as to call the troop movement an invasion.

â€œThe presence of foreign forces and interference in Bahrainâ€™s internal affairs is unacceptable and will further complicate the issue,â€ Ramin Mehmanparast, the Iranian Foreign Ministry spokesman told a news conference in Tehran, according to state-run media.

Even as predominantly Shiite Muslim Iran pursues a determined crackdown against dissent at home, Tehran has supported the protests led by the Shiite majority in Bahrain.

â€œPeople have some legitimate demands and they are expressing them peacefully,â€ Mr. Memanparast said. â€œIt should not be responded to violently.â€

He added: â€œWe expect their demands be fulfilled through correct means.â€

Iranâ€™s response â€” while anticipated â€” showed the depth of the rivalry across the Persian Gulf in a contest that has far-reaching consequences in many parts of the Middle East. Saudi Arabia has been watching uneasily as Bahrainâ€™s Shiite majority has staged weeks of protests against a Sunni monarchy, fearing that if the protesters prevailed, Iran, Saudi Arabiaâ€™s bitter regional rival, could expand its influence and inspire unrest elsewhere.

On Monday, about 2,000 troops â€” 1,200 from Saudi Arabia and 800 from the United Arab Emirates â€” entered Bahrain as part of a force operating under the aegis of the Gulf Cooperation Council, a six-nation regional coalition of Sunni rulers that has grown increasingly anxious over the sustained challenge to Bahrainâ€™s king, Hamad bin Isa al-Khalifa. â€œThis is the initial phase,â€ a Saudi official said. â€œBahrain will get whatever assistance it needs. Itâ€™s open-ended.â€

The decision is the first time the council has used collective military action to help suppress a popular revolt â€” in this case a Shiite popular revolt. It was rejected by the opposition, and by Iran, as an â€œoccupation.â€ The troops entered Bahrain at an especially combustible moment in the standoff between protesters and the monarchy. In recent days protesters have begun to move from the encampment in Pearl Square, the symbolic center of the nation, to the actual seat of power and influence, the Royal Court and the financial district. As the troops moved in, protesters controlled the main highway and said they were determined not to leave.

â€œWe donâ€™t know what is going to happen,â€ Jassim Hussein Ali, a member of the opposition Wefaq party and a former member of Parliament, said in a phone interview. â€œBahrain is heading toward major problems, anarchy. This is an occupation, and this is not welcome.â€

Rasool Nafisi, an academic and Iran expert based in Virginia, said:

â€œNow that the Saudis have gone in, they may spur a similar reaction from Iran, and Bahrain becomes a battleground between Saudi and Iran.

This may prolong the conflict rather than put an end to it, and make it an international event rather than a local uprising.â€

An adviser to the United States government, who spoke on the condition of anonymity because he was not authorized to talk to the news media, agreed. â€œIranâ€™s preference was not to get engaged because the flow of events was in their direction,â€ he said. â€œIf the Saudi intervention changes the calculus, they will be more aggressive.â€

Though Bahrain said it had invited the force, the Saudi presence highlights the degree to which the kingdom has become concerned over Iranâ€™s growing regional influence, and demonstrates that the Saudi monarchy has drawn the line at its back door. Oil-rich Saudi Arabia, a close ally of Washington, has traditionally preferred to operate in the shadows through checkbook diplomacy. It has long provided an economic lifeline to Bahrain.

But it now finds itself largely standing alone to face Iran since its most important ally in that fight, Hosni Mubarak of Egypt, has been ousted in a popular uprising. Iranâ€™s ally, Hezbollah, recently toppled the Saudi-backed government of Lebanon â€” a symbol of its regional might and Saudi Arabiaâ€™s diminishing clout.

But Bahrain is right at Saudi Arabiaâ€™s eastern border, where the kingdoms are connected by a causeway.

The Gulf Cooperation Council was clearly alarmed at the prospect of a Shiite political victory in Bahrain, fearing that it would inspire restive Shiite populations in Saudi Arabia and Kuwait to protest as well. The majority of the population in Saudi Arabiaâ€™s oil-rich eastern provinces is Shiite, and there have already been small protests there.

â€œIf the opposition in Bahrain wins, then Saudi loses,â€ said Mustafa el-Labbad, director of Al Sharq Center for Regional and Strategic Studies in Cairo. â€œIn this regional context, the decision to move troops into Bahrain is not to help the monarchy of Bahrain, but to help Saudi Arabia itself .â€

The Bahrain government said that it had invited the force in to help restore and preserve public order. The United States â€” which has continued to back the monarchy â€” said Monday that the move was not an occupation. The United States has long been allied with Bahrainâ€™s royal family and has based the Navyâ€™s Fifth Fleet in Bahrain for many years.

Though the United States eventually sided with the demonstrators in Egypt, in Bahrain it has instead supported the leadership while calling for restraint and democratic change. The Saudi official said the United States was informed Sunday that the Saudi troops would enter Bahrain on Monday.

Saudi and council officials said the military forces would not engage with the demonstrators, but would protect infrastructure, government offices and industries, even though the protests had largely been peaceful. The mobilization would allow Bahrain to free up its own police and military forces to deal with the demonstrators, the officials said.

The Gulf Cooperation Council â€œforces are not there to kill people,â€ said a Saudi official, speaking on the condition of anonymity because he was not authorized to talk to the press. â€œThis is a G.C.C. decision; we do not violate international law.â€

But the officials also acknowledged that it was a message to Iran.

â€œThere is no doubt Iran is involved,â€ said the official, though no proof has been offered that Iran has had anything to do with the political unrest.

Political analysts said that it was likely that the United States did not object to the deployment in part because it, too, saw a weakened monarchy as a net benefit to Iran at a time when the United States wants to move troops out of Iraq, where Iran has already established an influence.

The White House issued a statement calling on â€œthe government of Bahrain to pursue a peaceful and meaningful dialogue with the oppositionâ€ and for the â€œG.C.C. partners to show restraint.â€ Assistant Secretary of State for Near Eastern Affairs Jeffrey Feltman traveled to Bahrain on Tuesday to meet with â€œgovernment and non-governmental leaders,â€ according to a statement from the American Embassy, â€œin order to support a made-in-Bahrain approach to the political, social, and economic challenges that Bahrain faces.â€

The military force is one part of a Gulf Cooperation Council effort to try to contain the crisis in Bahrain that broke out Feb. 14, when young people called for a Day of Rage, fashioned after events in Egypt and Tunisia. The police and then the army killed seven demonstrators, leading Washington to press Bahrain to remove its forces from the street.

The royal family allowed thousands of demonstrators to camp at Pearl Square. It freed some political prisoners, allowed an exiled opposition leader to return and reshuffled the cabinet. And it called for a national dialogue.

But the concessions â€” after the killings â€” seemed to embolden a movement that went from calling for a true constitutional monarchy to demanding the downfall of the monarchy. The monarchy has said it will consider instituting a fairly elected Parliament, but it insisted that the first step would be opening a national dialogue â€” a position the opposition has rejected, though it was unclear whether the protesters were speaking with one voice.

The council moved troops in after deciding earlier to help prop up the king with a contribution of $10 billion over 10 years, and said that it might increase that figure. But if the goal was to intimidate Iran, or the protesters, that clearly was not the first response.

Bahrainâ€™s opposition groups issued a statement: â€œWe consider the entry of any soldier or military machinery into the Kingdom of Bahrainâ€™s air, sea or land territories a blatant occupation.â€